The present invention relates to the fractionation of air by selective adsorption, and is particularly directed to a pressure swing adsorption system obtaining enhanced recovery of oxygen from ambient air.
Fractionation of air by pressure swing adsorption essentially consists of two key operations: an adsorption step, where the compressed feed air is contacted with a sorbent capable of selectively adsorbing one of the main constituents of air, and a regeneration step, where the adsorbed components are removed from the sorbent so that it can be reused. Commonly, alumino-silicate zeolites, which selectively adsorb N.sub.2 from air, are employed as sorbents, thereby producing an O.sub.2 enriched air stream during the adsorption step.
The common methods for sorbent regeneration are (i) desorption by pressure reduction and (ii) desorption by elution or purging. Depressurization at the end of the sorption stroke reduces the superatmospheric pressure inside the sorption columns causing desorption of the sorbed components along with removal of the void gases from the column. The purging step following depressurization consists of eluting the column with a gas stream rich in the non-adsorbed species of the air (usually O.sub.2 enriched product gas) which reduces the partial pressure of the sorbed components in the voids of the column causing further desorption. Unfortunately, O.sub.2 is lost by both of these regeneration procedures, thereby lowering the recovery of that species in the oxygen-enriched product gas.
Among the numerous systems found in the patent literature for production of an oxygen-enriched product gas from air by pressure swing adsorption (PSA) are those described in U.S. Pat. Nos. 2,944,627; 3,564,816; 3,636,679; 3,717,974 which utilize zeolite molecular sieves as selective adsorbents for nitrogen-oxygen separation. Also, it is known to employ a pre-treatment section comprising one or more adsorbent beds for removal of water and CO.sub.2 from the feed prior to contact with the main adsorbent bed. Typical patents employing this feature include: U.S. Pat. Nos. 2,944,627; 3,140,931; 3,533,221; 3,796,022; 3,797,201; 3,957,463; 4,013,429. However, in these prior art systems, the air introduced into the main adsorption beds is at about ambient temperature, while a variation is proposed in U.S. Pat. No. 3,973,931. The latter patent discloses that, where the water-carbon dioxide impurities of the ambient air are removed by adsorbing these impurities in the feed inlet end of the same adsorbent column where the nitrogen-oxygen separation is carried out, an undesired cooling of that front section takes place. This is described as being responsible for the observed lower oxygen recovery of these processes in actual large scale application. Consequently, it is proposed in this patent to supply heat from an external source to only the inlet end section of the column sufficient to maintain the inlet end at least 20.degree. F. (11.degree. C.) warmer than without heating, but below 175.degree. F. (79.4.degree. C.). Similarly, a companion patent, U.S. Pat. No. 4,026,680, concerned with the same general problem of reduced temperature at the inlet end of the adsorption bed, uses metallic elements embedded in the column. This procedure is stated to have helped reduce the problem of the inlet end temperature depression and to result in improved oxygen recovery.
In the preliminary studies leading to the development of the present invention, two important characteristics of pressure swing adsorption systems for air separation were investigated. These were: (1) the temperature effect on the N.sub.2 capacity of the adsorbent during adsorption from air at superatmospheric pressure and (2) the temperature effect on the quantity of oxygen purge gas needed for cleaning the adsorbent column after depressurizing the column to a near ambient pressure level. A synthetic mordenite molecular sieve was used in the tests conducted in this investigation.
It was found from these tests that both the N.sub.2 capacity and the O.sub.2 purge requirements decreased as the base column temperature increased, as was expected. However, there was a significant difference in the temperature coefficients of these two properties, not heretofore appreciated. That is, the temperature coefficient for the oxygen purge gas was found to be 4 to 10 times that for the nitrogen capacity, depending upon the conditions of operation.
Based on the foregoing exploratory tests, it was discovered that the performance of a pressure swing adsorption process (PSA) for air separation could be improved by operating the system at an elevated temperature throughout the columns. While in such operation a larger inventory of adsorbent may be required due to decrease in the adsorption capacity, the O.sub.2 loss during the elution of the bed would be substantially reduced. In addition, it was found that the oxygen loss during the depressurization of the columns could also be reduced when operating these at an elevated base temperature throughout the bed and during the entire operating cycle. Also, it was discovered that the greater oxygen recovery would more than offset any anticipated increase in the capital cost, and thereby substantially improve the total economics of generating product oxygen by pressure swing adsorption, such for example, as an overall cost decrease in the order of 10 to 30%.
In addition to obtaining higher oxygen recovery, operation of the air fractionation process in accordance with the present invention provides another important advantage. By operating the pressure swing air fractionation system during the entire cycle at substantially the same elevated temperature at which the hot feed air is introduced thereto, the pretreatment of the feed air for removal of water and carbon dioxide prior to oxygen-nitrogen separation can be conveniently carried out by a thermal swing adsorption scheme because the hot desorbed nitrogen and the hot waste purge gas from the N.sub.2 --O.sub.2 separation can be used for thermal regeneration of the pretreatment columns. Ordinarily, the adsorbent in the front section of the main N.sub.2 --O.sub.2 separation column is utilized as a trap for H.sub.2 O and CO.sub.2, and a cyclic regeneration of this section is achieved during the depressurization and purging steps for the regeneration of the N.sub.2 --O.sub.2 section. Such procedure for regeneration of the H.sub.2 0--CO.sub.2 section is not very efficient because most of the desorption of water and carbon dioxide is achieved by the " purging effect" of the H.sub.2 O--CO.sub.2 free gases from the N.sub.2 --O.sub.2 separator section. A portion of the oxygen-enriched purge gas is thus often consumed only to clean the pretreatment section, particularly to remove water, and thereby decreases the amount of oxygen which can be recoverd as a dry product.
Thus, by operating at elevated temperature and in accordance with the particular steps of the present invention, one can judiciously utilize the heat of compression to supply the energy required for operation at the desired elevated temperature, as well as the energy needed for thermal swing removal of the H.sub.2 O and CO.sub.2 impurities in the feed air, such that the requirement for an external heat source is eliminated.